Turbulence Characteristics in a Gradual Channel Transition

By combining detailed field and laboratory tests, this study has put forth a method for determining the incipient conditions for the fluvial erosion (Type I) of cohesive bank sediments in a natural gravel bed stream, Union Flat, found in the Palouse region of WA. Union Flat is characterized by a sequence of cross-sectional irregularities such as channel expansions and constrictions. In particular this study contributes: 1) A laboratory procedure for determining the critical stress for fluvial erosion (Type I) of cohesive bank sediments. 2) A method for determining the fluid stresses present on the bank by accounting for turbulence and secondary currents due to the cross-sectional irregularities. 3) An improved understanding of the turbulence characteristics found in a gradual expansion of Union Flat. Furthermore, the analysis of these measurements shows that the presence of the secondary currents increases the magnitude of the side wall shear stress. The side wall shear stress obtains values that are many times, higher in magnitude than the corresponding stress for uniform flow conditions. The results also show that even for a low magnitude flow event the local side wall shear stress τ_s obtains values that are almost 2–3 times greater than the depth-averaged τ¯_s value. It is suggested that use of the τ¯_s may be a good approximation of the fluid shear stress magnitude for simple channel geometries but not for natural channels characterized by width expansions and constrictions. Perhaps knowledge of the ratio τ_s/τ¯_s over depth can provide a better measure for the commencement of fluvial erosion in channels with cross-sectional irregularities.